1. Field of the Invention
The present invention relates to a differential signal transmission circuit formed on an insulating layer of a double-sided flexible printed circuit board by a semi-additive method, and a method for manufacturing the same.
2. Description of the Related Art
In recent years, differential signal transmission technology that transfers data using a pair of signal lines has been widely employed as a high-speed signal transmission technology. Differential signal transmission is configured to transmit signals having phases opposite to each other, by two signal lines configuring a differential signal transmission circuit, hence has excellent noise immunity compared to single-end transmission, and can transmit data at high speed with a small signal amplitude.
Such differential signal transmission has an on-circuit characteristic impedance (differential impedance) Zdiff defined according to various standards. This differential impedance Zdiff is obtained by various elements, for example: distance between the two signal lines or circuit width; distance between the signal lines and a ground (GND) line; distance between a circuit formed on a surface on an opposite side to the signal lines sandwiching an insulating layer, and the signal lines; and so on.
Moreover, in differential signal transmission, in order to further increase noise immunity and so on while keeping pace with, for example, miniaturization of portable information terminals, increase in amounts of data transmitted, and so on, it is generally considered desirable that the two signal lines configuring the differential signal transmission circuit are disposed close to each other.
This is because disposing the two signal lines close to each other allows configuration of a closed system where many of magnetic field lines generated by a current flowing in one of the signal lines terminate in the other of the signal lines, whereby tolerance to noise from external can be increased. Moreover, in this case, thickness of the two signal lines is preferably made as constant as possible. This is because, if thickness of the two signal lines differs, symmetry of a transmission path breaks down and a differential signal of the two signal lines is converted to common mode noise, whereby radiation of electromagnetic waves or malfunction occur.
Now, in the case of trying to form a differential signal transmission circuit on a flexible printed circuit board, there is a structure where thickness of a base film (insulating layer) is extremely thin compared to thickness of a base material in a rigid substrate, hence capacitance C due to coupling between conductors formed on both of front and reverse surfaces sandwiching the insulating layer increases, whereby the differential impedance Zdiff lowers.
As a result, in order to secure a differential impedance Zdiff defined according to a standard, it is necessary to set design conditions such that the differential impedance Zdiff increases more than for design conditions in a rigid substrate. The following methods exist for increasing the differential impedance Zdiff in a differential signal transmission circuit in a flexible printed circuit board.
Namely, any of the methods of: (1) narrowing a line width of the two signal lines; (2) widening a spacing of the two signal lines; and (3) widening a distance between the signal line and the GND line.
At the same time, in recent years, this kind of differential signal transmission circuit has increasingly come to be formed by a semi-additive method. The semi-additive method is a method where, for example, having formed a seed layer, by non-electrolytic plating, on a base film that has undergone processing to roughen a surface of the base film, a plating resist is formed to perform an electrolytic plating, and the plating resist and the seed layer are removed to form a circuit pattern. The semi-additive method is widely employed when forming a minute circuit with high precision.
However, there is a problem that when the semi-additive method is employed, a variation in current density occurs due to a difference in width of wiring lines formed in a process of a plating layer growing on the seed layer, and there is a tendency for the plating to be formed thickly in broad places of the wiring lines and thinly in narrow places of the wiring lines, giving rise to variation in thickness of a wiring line layer.
In order to try to avoid such a problem, a method for forming a differential signal transmission circuit configured to have a circuit thickness which is constant, and configured to have a dummy pattern formed in a close vicinity of the circuit pattern configuring the differential signal transmission circuit to thereby suppress variation in current density during electrolytic plating, is known (refer to, for example, Examined Japanese Patent Application Publication No. JP 2000-323525 A and Unexamined Japanese Patent Application Publication No. JP 2007-149737 A).